Drilling with casing is gaining popularity as a method for drilling wherein the casing is used as the drill string and drilling conduit and, after drilling, the casing remains downhole to act as the wellbore liner. A drilling assembly, often including at least a drill bit and one or more hole enlargement tools such as, for example, an underreamer, is used which drills a borehole of sufficient diameter to accommodate the casing. The drilling assembly is deployed on the advancing end of the casing. The drilling assembly can be retractable and/or removable through the casing.
Drilling with casing has been tested for drilling vertical, straight and deviated wellbores.
Another form of drilling with casing is termed liner drilling. In liner drilling, the drilling assemblies operate and advance to extend the borehole while being mounted on the end of a section of liner. The liner is connected to surface by a length of drill pipe or additional casing.
When drilling with casing, it may be desirable from time to time during drilling and/or at the end of the drilling operation to retrieve the drilling assembly to surface. This is accomplished by tripping the drilling assembly up though the casing. Various tripping methods have been employed such as by running in pipe strings, wireline, coiled tubing, etc. to engage the drilling assembly and pull it to surface. Alternately, in other methods, a fluid conveyed dart may be used to manipulate the drilling assembly to release it from the drilling string so that the drilling assembly can be tripped to surface. By use of a dart, no rigid work string is required to be used to move the dart along even a deviated or horizontal drill string and circulation of drilling fluid can, if desired, continue during substantially the entire conveying and disengaging operation, with the exception of a short period during which the drill string is opened to introduce the dart. After manipulating the drilling string, a string may be used to pull the drilling assembly to surface. Thus, the dart may be connected to a line that is pulled behind the dart or may be free of any connection but a string is run in afterward to engage the drilling assembly and pull it to surface.
There is also interest in using reverse circulation move a drilling assembly to surface. The retrieval of a drilling assembly by reverse circulation proceeds wherein after the drilling assembly is disengaged from the drill string, drilling fluid is pumped down through the annulus between the drill string and the borehole to act against the drilling assembly and force it up through the drill string toward surface. Continued reverse circulation can lift the drilling assembly so that it can be retrieved at surface.